On the lower wooden block, drill a hole and insert a fairly large screw. It should have flat end in order not to damage the support for the lamp (generally a bed or a shelf).

It would be difficult to do a thread in the wood (however, you can try). I inserted a nut into the wood to provide for the threaded support.

Inserting the nut is very tricky. As I have no special tools, I drilled a counter bore a little smaller than the nut. Then I hammered the nut into this bore.

The hammering is the trickiest part. If it is too light, the nut may move eventually, if it is too hard, the wood may break.